Improving the ability to predict spatial and temporal variations in K bioavailability is critical to optimize fertilizer use. This study assessed the effect of K fertilization and residual K fertilizer on plant available K and yield responses in a corn-soybean rotation grown in a tilled Tracy sandy loam and no-till Toronto-Millbrook silt loam in north-central and west-central IN, respectively. Cumulative fertilizer rates applied ranged from 0 to 900 kg/ha (1997-2002). Following 2002, no K fertilizer was applied. Exchangeable soil K (1 mol ammonium-extractable (NH4-ext)) was determined at depths 0 to 10 cm, and 10 to 20 cm at the V6 and R1/R2 growth stage for corn and soybean, respectively (annually 1997-2003; 2009-present). Grain yield was determined every year until present.

During the build-up phase, fertilizer application rates had a significant effect on NH4-ext 9 out of 12 times (p<0.01) for the Tracy soil, and 11 out of 12 times (p<0.01) in the Toronto-Millbrook. Soil K was highly stratified with depth at both sites with the 0- to 10-cm soil layer having significantly higher NH4-ext levels than the 10- to 20-cm layer for all site-years. Yield response to fertilization was significant only 3 out of 12, and 1 out of 12 times for the same sites. After K applications ceased, a significant fertilizer rate effect on NH4-ext was observed in 2003 and 2010 for both sites, with significantly lower NH4-ext values in 2010. Stratification was highly significant with ratios of 1.2 to 1.5 and 1.2 to 1.9 in 2010 for Toronto-Millbrook and Tracy soils, respectively. Treatment differences in yield were observed 7 out of 16 times and 4 out of 16 for the same sites during 2003-2010. The eight years of K mining (2003-2010) reduced NH4-ext K to 82 and 58 mg/Kg for the 0- to 10-cm layer, and 68 and 38 mg/Kg for the 10- to 20-cm depth increment for the Toronto-Millbrook and Tracy soils, respectively; far below published critical values. In 2010, only soybean yields were significantly reduced where NH4-ext values were the lowest. Corn yields were not affected at any site, which indicates that mean NH4-ext K values of 114 and 79 mg/Kg in the 0- to 10-cm layer were able to maintain satisfactory yields for the Toronto-Millbrook and Tracy soils, respectively.